During a data communication between signal transmission devices in a communication system, if an impulse noise is applied to a transmission line (or a transmission cable) at the time when a communication frame is outputted, frame loss occurs in the communication frame, and thus degrading transmission efficiency.
As an example, in Ethernet, a user can set the data length of communication frames to optional length. In such a communication system, if communication is performed with the data length of communication frames set to be long, transmission efficiency degradation for when frame loss occurs due to an impulse noise becomes noticeable and brings disadvantages.
Contrary to above, if communication is performed with the data length set to be very short, transmission efficiency degradation due to impulse noises may decrease. However, transmission efficiency degrades due to the overhead of control fields such as frame headers, and thus being disadvantageous.
In general, in a communication system that performs signal transmission with impulse noises, there is an intermediate data length being capable of achieving the best transmission efficiency. This data length of the best transmission efficiency depends on the occurrence intervals of impulse noises.
Here, as shown in FIG. 4, a communication frame 4 is composed of bit fields including a preamble & header 41, data 42, and an FCS (Frame Check Sequence) 43. A user can optionally set the length of the data 42. It is assumed that the frequency of occurrence of an impulse noise 5 is distributed randomly around a certain average interval, and frame loss occurs in a communication frame 4 outputted at a time when overlapping with an impulse noise 5. Note that numeral 44 denotes a frame gap between communication frames 4.
FIG. 5 shows an example of results of studying, by numerical analysis, the data length being capable of achieving the best transmission efficiency when communication frames 4 shown in FIG. 4 are used. The horizontal axis of FIG. 5 represents the data length of communication frames 4 and the vertical axis represents effective transmission efficiency. Note that the communication rate is assumed to be 125 Mbps, and the frequencies of occurrence of an impulse noise 5 are 40 μs, 80 μs, and 160 μs.
In the analysis results shown in FIG. 5, the best transmission efficiency is achieved in the case of intermediate data lengths of 200 to 300 bytes (region A). Note that, in a portion where the data length is long (region B), the transmission efficiency decreases due to the application of impulse noises 5. In addition, in a portion where the data length is very short (region C), the transmission efficiency decreases due to the overhead of control fields such as headers.
Based on relations such as that described above, in order to improve the transmission efficiency, techniques for optimally setting the data length are disclosed (See Patent Literatures 1 and 2, for example). In these conventional techniques, as shown in FIG. 6, a detecting unit 100 detects a frame error by using an FCS 43 provided to a communication frame 4, and a data length changer 101 varies the data length to an optimal length based on the detection result.